1. harder to use than windows, enough said.
2. compatibility issues - ati drivers, windows drivers, programs schools and business use runs on proprietary Windows only, etc, etc, etc, etc.
An os needs rock solid stability and be compatible and perfectly stable with the hundreds of thousands of different pc devices & configurations out there to be used by 1/6th of the population, and not only make it compatible but make it easy to use is a gargantuan task.
Such a gargantuan task that Vista cost 6 billion dollars to develop.
Linux is developed by geniuses, and to me surprising its as good as it is already, but to think an open source project can be an easy to use os with compatibility for one sixth of the planet is fantasy.